The invention relates to a system and method for recording data onto an optical disc, and more particularly, to a system and method for printing a visible image onto an optical disc through tuning a driving signal of an optical pick-up unit.
In addition to recording normal user data, optical disc recording systems are developed to be able to form visible images on the optical disc. Many conventional optical disc recording systems have been developed to carry out the visible image printing. Please refer to FIG. 1. FIG. 1 is a first optical disc recording system 100 according to the related art. The optical disc recording system 100 is configured to print a visible image onto an optical disc 101, such as a CD-R, CD-RW, DVD-R, DVD+RW, DVD-RW, DVD-RAM or other recordable discs. The optical disc recording system 100 comprises an optical pick-up unit (OPU) 102, a spindle motor 104, a spindle controller 106, a draw data generator 108, and a laser driver 110. The draw data generator 108 is used for converting image data received from a host (not shown) into draw data and then outputting the draw data to the laser driver 110. Next, the laser driver 110 is operative to convert the incoming draw data into a driving signal used for driving the OPU 102. In other words, the laser driver 110 drives the OPU 102 in response to the draw data such that a laser beam is emitted at specific irradiation timing and a specific laser power level. Thus, the laser beam emitted from the OPU 102 is applied to the optical disc 101, thereby recording corresponding image data and forming a visible image thereon.
As shown in FIG. 1, the spindle motor 104 is controlled by the spindle controller 106 to rotate the optical disc 101. In this conventional optical disc recording system 100, the spindle controller 106 refers to a frequency generator (FG) signal FG_A, generated due to rotations of the spindle motor 104, to control the spindle motor 104 to rotate the optical disc 101 at a fixed rotation speed. In this way, the desired image is printed onto the optical disc 101 at a fixed printing speed accordingly.
However, it is difficult to make the optical disc 101 steadily rotate at a fixed rotation speed due to the characteristics of the spindle motor 104, such as the slow response of the spindle motor 104, or the imperfect characteristics of the loaded optical disc 101, such as the disc eccentric.
Because the mechanical adjustment applied to the spindle motor 104 is time-consuming, if an abrupt shock occurs, the spindle controller 106 becomes unable to control the spindle motor 104 to stabilize the rotation of the optical disc 101 in time. This unexpected disturbance will degrade the visible image quality. In other words, if random vibration occurs due to unideal characteristics of the spindle motor 104 and/or the optical disc 101, the conventional spindle control loop may fail to track the spindle rotation error caused by the random vibration promptly because of the low response of the spindle motor 104.
Please refer to FIG. 2. FIG. 2 is a diagram illustrating the first problem caused by the spindle rotation error. In FIG. 2, the top portion, FIG. 2(a), shows a visible image without spindle error present, while the bottom portion, FIG. 2(b), shows a visible image with random spindle error present. As shown in FIG. 2, since the spindle control loop is unable to track the transient vibration of each motor revolution cycle of the spindle motor 104 immediately, the induced blurred edges at both sides could degrade the quality of the visible image printed (recorded) on the optical disc 101 when perceptible to human eyes.
In addition to the above-mentioned random spindle error, the spindle rotation error also includes periodic variations. That is, the spindle rotation error is periodic to the rotation angles. The angle-periodic spindle error is usually caused by the unbalanced damping of the spindle motor 104, poor spindle bearing, the eccentric disc, or possible combinations thereof. Please refer to FIG. 3. FIG. 3 is a diagram illustrating the second problem caused by the spindle rotation error. In FIG. 3, the top portion, FIG. 3(a), shows a visible image without spindle error present, the middle portion, FIG. 3(b), shows a visible image with periodic spindle error present, and the bottom portion, FIG. 3(c), shows the rotation speed variation. As one can see, the average rotation speed is fixed at V1; however, the instant rotation speed varies from lowest speed V2 to highest speed V3 periodically, resulting in the perceivable image distortion. Because of the brightness or gray level of the printed image is in proportion to the laser power density per unit area of the recordable disc layer where the visible image is printed thereon. Therefore, different rotation speeds of the spindle motor 104 correspond to different laser power density. As a result, the image area corresponding to the highest rotation speed V3 has lowest brightness or gray level, while the image area corresponding to the lowest rotation speed V1 has highest brightness or gray level. In short, as shown in FIG. 3(b), because of the periodic rotation speed variation, the brightness or gray level of the printed image is not uniform accordingly. It should be noted that human eyes are much more sensitive to the periodic spindle error than the above-mentioned random spindle rotation error.
Facing aforementioned problems, the spindle control loop implemented in the conventional optical disc recording system 100 is unable to solve either of them. Additionally, as shown in FIG. 1, in the spindle control loop the spindle controller 106 uses the FG signal FG_A generated from the spindle motor 104 as a reference to control the spindle motor 104 has two major problems. The first problem is that the frequency of the FG signal FG_A is only several tens times higher than that of the spindle rotation. In a case where the optical disc 101 is rotated at a low rotation speed, the spindle motor 104 may not be well controlled since the feedback signal, the FG signal FG_A, has a quite low frequency. The second problem is that the periods of adjacent FG edges in the FG signal FG_A, detected from many Hall sensors, are not equidistant because of the inaccurate placement of these Hall sensors and the stator slots. As a result, it is not guaranteed that the edges of the FG signal FG_A are equidistant. Therefore, if the spindle controller 106 employs the phase detection means to control the spindle motor 104, the rotation speed control becomes inaccurate.
Please refer to FIG. 4 in conjunction with FIG. 5. FIG. 4 is a block diagram of one conventional embodiment of the spindle controller 106. FIG. 5 is a waveform diagram of an ideal FG signal FG_I, an actual FG signal FG_A and a reference FG signal FG_R. As shown in the circuit configuration of FIG. 4, the spindle controller 106 includes a frequency divider 112, a phase detector 114, and a control circuit 116, where the frequency divider 112 is used for dividing the incoming reference clock CLKref to generate a reference FG signal FG_R, the phase detector 114 is used for detecting the phase error between the reference FG signal FG_R and an actual FG signal FG_A, and the control circuit 116 is used for controlling the spindle motor 104 to tune the rotation speed according to the measured phase error from the phase detector 114. However, for simplicity, suppose that the actual FG signal FG_A has three FG edges FG0, FG1, FG2 per motor revolution of the spindle motor 104 and periods between two adjacent FG edges are not equidistant. As shown in FIG. 5, the phase error P1, detected by the phase detector 114, indicates that the FG edge FG1 leads the edge of the reference FG signal FG_R, meaning that the current rotation speed of the optical disc 101 is higher that the desired rotation speed. Next, the control circuit 116 will control the spindle motor 104 to slow down its spindle rotation speed. However, the following phase error P2, detected by the phase detector 114, indicates that the FG edge FG1 lags behind the edge of the reference FG signal FG_R, meaning that the current rotation speed of the optical disc 101 is lower that the desired rotation speed. Therefore, the control circuit 116 then controls the spindle motor 104 to speed up it spindle rotation speed. Due to the FG period unbalance, the spindle motor 104, however, is erroneously controlled.
To solve above problem caused by unbalanced FG periods, another conventional spindle controller configuration is disclosed. Please refer to FIG. 6 in conjunction with FIG. 7. FIG. 6 is a block diagram of another conventional embodiment of the spindle controller 106. FIG. 7 is a waveform diagram of an ideal FG signal FG_I, an actual FG signal FG_A, a frequency-divided FG signal FG_DIV, and a reference FG signal FG_R′. As shown in the circuit configuration of FIG. 6, the spindle controller 106 includes a counter 121, a frequency divider 122, a phase detector 124, and a control circuit 126, where the counter 121 is used for counting one FG period of an incoming actual FG signal FG_A to generate a frequency-divided FG signal FG_DIV, the frequency divider 122 is for dividing the incoming reference clock CLKref to generate a reference FG signal FG_R′, the phase detector 124 is used for detecting the phase error between the reference FG signal FG_R′ and the frequency-divided FG signal FG_DIV, and the control circuit 126 is used for controlling the spindle motor 104 to tune the spindle rotation speed according to the measured phase error from the phase detector 124. For simplicity, suppose that the actual FG signal FG_A has three FG edges FG0, FG1, FG2 per motor revolution of the spindle motor 104 and periods between two adjacent FG edges are not equidistant. As shown in FIG. 7, the counter 121 is configured to count the first FG edge FG0 to generate the frequency-divided FG signal FG_DIV, and the period of the reference FG signal FG_R′ is three times of the period of the aforementioned reference FG signal FG_R. Since only one of the FG edges within one motor revolution (e.g., the FG edge FG0) accounts for detecting the phase error, the above problem caused by unbalanced FG periods is solved. However, because the spindle controller 106 updates the control signal applied to the spindle motor 104 once per motor revolution, the spindle motor control efficiency and accuracy are greatly degraded.
Please refer to FIG. 8. FIG. 8 is a second optical disc recording system 200 according to the related art. The optical disc recording system 200 is configured to print a visible image onto an optical disc 201, such as a CD-R, CD-RW, DVD-R, DVD+RW, DVD-RW, DVD-RAM or other recordable discs. The optical disc recording system 200 comprises an optical pick-up unit (OPU) 202, a spindle motor 204, a spindle controller 206, a draw data generator 208, a laser driver 210, an FG-PLL circuit 212, and a position detector 214. The function and operation of the OPU 202, the spindle motor 204, and the laser driver 210 are almost the same as that of the OPU 102, the spindle motor 104, and the laser driver 110 shown in FIG. 1. In this conventional optical disc recording system 200, the FG-PLL circuit 212 is a phase-locked loop implemented to generate a pulse signal by locking to phase of each FG edge in the FG signal FG_A outputted from the spindle motor 204 per predetermined rotation angle. The position detector 214 receives the pulse signal outputted from the FG-PLL circuit 212 and then determines a position of the OPU 202 in the disc circumferential direction according to the received pulse signal. Next, the draw data generator 208 refers to the position of the OPU 202 to convert an image data into a draw data accordingly. As shown in FIG. 8, the spindle controller 206 also receives the pulse signal generated from the FG-PLL circuit 212, and controls the spindle rotation speed of the spindle motor 204 according to the received pulse signal. As mentioned above, the FG-PLL circuit 212 is used for generating an additional signal for controlling spindle motor by locking to the phase of each FG edge. However, the FG-PLL circuit 212 provides a pulse signal synchronous to an average phase of the FG edges instead of actual phases of the FG edges. As a result, the spindle control loop adopted by the optical disc recording system 200 is only capable of making the spindle rotation speed stable using the steady-state phase information, but it fails to solve the aforementioned problems, edge blurring caused by random spindle error and the inconstant brightness or gray level caused by periodic spindle error.
Please refer to FIG. 9. FIG. 9 is a third optical disc recording system 300 according to the related art. The optical disc recording system 300 is configured to print a visible image onto an optical disc 301, such as a CD-R, CD-RW, DVD-R, DVD+RW, DVD-RW, DVD-RAM or other recordable discs. The optical disc recording system 300 comprises an optical pick-up unit (OPU) 302, a spindle motor 304, a spindle controller 306, a draw data generator 308, a laser driver 310, and a sensor 305. The function and operation of the OPU 302, the spindle motor 304, and the laser driver 310 are almost the same as that of the OPU 102, the spindle motor 104, and the laser driver 110 shown in FIG. 1. In this conventional optical disc recording system 300, the optical disc 301 has rotation marks disposed thereon, where the sensor 305 is implemented to sense the rotation marks. That is, each time a rotation mark passes the sensor 305, the sensor 305 generates a pulse. The spindle controller 306 then uses the pulses outputted from the sensor 305 to control the spindle motor 304. Because the rotation marks are precisely formed on the optical disc 301, the phase information carried by each pulse generated from the sensor 305 can be used by the spindle controller 306 to offer accurate spindle motor control. Additionally, there are many rotation marks disposed on the optical disc 301. It is clear that the frequency of the pulse signal generated from the sensor 305 is much higher than that of the FG signal FG_A generated in response to predetermined rotation angles of the spindle motor 304, allowing the spindle motor 306 to be well controlled to maintain at stable spindle rotation speed. However, even though the rotation error is precisely detected using the sensor 305 to sense the rotation marks, the detected rotation error cannot be eliminated instantly because of the low response of the spindle motor 304. As a result, the spindle control loop adopted by the optical disc recording system 300 still fails to solve the aforementioned problems, edge blurring caused by random spindle error and the inconstant brightness or gray level caused by periodic spindle error.